1. Field of the Invention
The present invention relates to a semiconductor device. More particularly, the present invention relates to a capacitor of a semiconductor device and a memory device using the same.
2. Description of the Related Art
A semiconductor device, such as a memory device, includes a transistor and a capacitor. A function of the capacitor is to maintain data in a normal state for a relatively long period of time. To accomplish this function, the capacitor has a capacitance that is greater than a certain value.
As a degree of integration of a memory device increases, an area in which a capacitor may be formed decreases. Regardless of this decrease in area, a capacitance of a capacitor should be increased, or at least should not be decreased.
In view of this situation, research for a capacitor that can be applied to a highly integrated memory device has been conducted. In this research, a method of thinning a dielectric layer, a method of utilizing ferroelectric substances, and a method of combining the two methods have been used.
FIG. 1 illustrates a cross-sectional view of a capacitor included in a semiconductor device according to the prior art.
Referring to FIG. 1, a conventional capacitor includes a lower electrode 10, a hafnium oxide (HfO2) layer 12, and an upper electrode 14.
As the degree of integration of the memory device increases, a leakage current of the conventional capacitor increases.
Accordingly, the HfO2 layer 12, which is used as a dielectric layer, should be as thin as possible. However, as the HfO2 layer 12 is made thinner, e.g., around 4.5 nm thick, the leakage current of the capacitor significantly increases. Thus, the memory device including the conventional capacitor does not function properly.
FIG. 2 is a graph of a leakage current density of the capacitor of FIG. 1 versus voltage according to a thickness of the HfO2 layer 12.
Referring to FIG. 2, first and second curves G1 and G2 represent the characteristics of the leakage current when the HfO2 layer 12 has a thickness much greater than 4.5 nm. Third and fourth curves G3 and G4 represent the characteristics of the leakage current when the HfO2 layer 12 has a thickness of about 4.5 nm. Further, the first and third curves G1 and G3 each represent leakage current density when the HfO2 layer 12 is deposited to a thickness less than a target thickness. The second and fourth curves G2 and G4 each represent the leakage current density when the HfO2 layer 12 is deposited to the target thickness. Comparing the first and second curves G1 and G2 with the third and fourth curves G3 and G4, it may be seen that the leakage current of the capacitor significantly increases when the HfO2 layer 12 has a thickness of about 4.5 nm.
FIG. 3 illustrates a cross-sectional view of a conventional capacitor having a relatively complicated structure including a ferroelectric layer. FIG. 4 is a graph illustrating a ratio of atoms included in the ferroelectric layer of FIG. 3, measured at a plurality of points in the capacitor of FIG. 3;
In an alternative conventional capacitor, a ferroelectric layer M3 having multi-components, such as an STO (SrTiO3) layer, may be used, instead of the HfO2 layer 12. The alternative conventional capacitor shown in FIG. 3 includes a silicon oxide (SiO2) layer M1, a lower electrode M2, and the ferroelectric layer M3. An upper electrode is not shown in the capacitor of FIG. 3.
When the capacitor has a complicated structure, like the capacitor in FIG. 3, a ratio of positive ions in the ferroelectric layer M3 changes and an electrical characteristic of the capacitor is degraded. Referring to FIG. 4, fifth curve G5 represents a ratio of Sr to Ti in the ferroelectric layer M3, e.g., the STO layer, which is measured at a plurality of points 1–9 in the capacitor of FIG. 3.
According to the fifth curve G5, a content of Sr at a top surface of the ferroelectric layer M3, i.e., at point 1, is considerably greater than that of Ti. The content of Sr at a bottom surface of the ferroelectric layer M3, i.e., at point 9, however, is slightly less than that of Ti.